Electronic devices, such as mobile phones, can include external connectors for interfacing with peripherals. One type of external connector is an audio jack, e.g., an audio socket. For example, the audio jack of a mobile device can be coupled with an audio plug of a credit card reader. Once coupled, the mobile device and the card reader can communicate data with one another by transmitting and receiving electrical signals through the audio plug. Other types of external connectors include USB ports and 30-pin connectors. Some electronic devices include wireless communication interfaces, e.g., wireless LAN and Bluetooth.